Etymology Dictionary

FAITH,  belief.  (F.,—L.)   The final -th answers to -d in O.F. feid, the change to th being made to render it analogous in form with truth, ruth, wealth, health, and other similar sbs.   β. M.E. feiþ, feith, feyth; as well as fey.   The earliest example of the spelling feyth is perhaps in Havelok, l. 2853; fey occurs in the same poem, ll. 255, 1666.—O.F. fei, feid; also foi, foit.—Lat. fidem, acc. of fides, faith. + Gk. πίστις, faith; πείθειν, to persuade; πέποιθα, I trust.—✔BHIDH, to unite; weakened from ✔BHADH, fuller form ✔BHANDH, to bind.   See Bind.   See Curtius, i. 325.   Der. faith-ful, faith-ful-ly, faith-ful-ness; faith-less, faith-less-ly, faith-less-ness.   From the same root are fid-el-i-ty, af-fi-ance, con-fide, de-fy, dif-fid-ent, per-fid-y. [†]

ADDENDA

The M.E. form fey is due to O.F. fei, whilst the M.E. form feith represents the O.F. feid, which is the earliest O.F. form, the d being due to L. acc. fidem.   On the final -th, see H. Nicol's article in The Academy, no. 435, Sept. 4, 1880, p. 173, where this view is maintained.   On the other hand, the fact that -th is a common ending for abstract nouns (such as health, wealth) may account for the change from d to th.